As illustrated in FIG. 3, the bus interface logic 54a, . . . , 54n of each subsystem module, runs off the bus clock signal (bclk) 74 which is generated by central bus interface block 43 and routinely derived from the CPU processor clock signal, albeit at a slower rate than the CPU clock, and each of the bus interface logic units 54n, continuously monitors activity, such as the occurrence of an add